After a poor performance in April, the French left has unified in a coalition for what its leader Jean-Luc Melenchon called “the third round” of the presidential elections.
Elections for the 577 seats in the lower house National Assembly are a two-round process. The shape of the new parliament will become clear only after the second round, a week later, on June 19.
The ballots provide a conclusion to April's presidential election, when Mr Macron won a second term and pledged a transformative era after a first mandate dominated by protests, the coronavirus pandemic and Russia's war against Ukraine.
Polls opened in mainland France at 8am after voters in overseas territories cast ballots earlier in the weekend.
Opinion polls show the president's centrist alliance, Ensemble (Together), and Melenchon's Nupes coalition of left, socialists, communists and Greens neck-and-neck in the popular vote.
But France's constituency-based parliamentary system and the two-round election means that the seat breakdown will be another matter, and much will depend on turnout in the second round.
The abstention rate is predicted to be more than 50 per cent in the first round, in what would be a record for elections already marked by low participation in recent years.
If the president's alliance retains an overall majority, Mr Macron will be able to carry on governing as before.
Falling short could prompt messy bill-by-bill deals with right-wing parties in Parliament or an unwanted Cabinet reshuffle.
A win by the left-wing alliance — seen as unlikely by analysts but not impossible — would be trouble for Mr Macron.
It would raise the spectre of a clunky “cohabitation”, where the prime minister and president hail from different factions, which has paralysed French politics in the past.
The most recent example was from 1997 to 2002, when right-wing president Jacques Chirac ruled in tandem with Socialist Lionel Jospin as Prime Minister.
Socialist president Francois Mitterand twice had to cohabit with right-wingers: with Chirac in a famously fractious relationship and then with Edouard Balladur.
Mr Melenchon, a former Marxist, has already made clear his ambition to become prime minister and stymie Mr Macron's plan to raise the French retirement age, although the president would retain control over foreign policy.
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Mr Macron acknowledged the stakes were high, warning France against choosing “extremes” that would add “crisis to crisis”.
“If the presidential election is crucial, the legislative election is decisive,” he said on a visit to the rural Tarn region, calling for a “strong and clear majority”.
Polls have indicated that Mr Macron's alliance is expected to win the largest number of seats but is by no means assured of getting over the line of 289 for an absolute majority.